CN101448580B - Plasma etch and photoresist strip process with chamber de-fluorination and wafer de-fluorination steps - Google Patents

Abstract

The present invention discloses a plasm etch process includes a plasma etch step performed with a photoresist mask on a workpiece using a polymerizing etch process gas that produces in the plasma polymerizing species which accumulate as a protective polymer layer on the surface of said photoresist mask during the etch step, the process including the following steps performed in the same chamber after the etch step and prior to removing the photoresist mask: (a) removing residue of the type including polymer material from chamber surfaces including a ceiling of said chamber, by coupling RF plasma source power into the chamber while coupling substantially no RF plasma bias power into the chamber, and introducing a hydrogen-containing gas into the chamber, until said residue is removed from the chamber surfaces; (b) removing the protective polymer layer from the surface of the photoresist mask, by coupling RF plasma bias power into the chamber while coupling substantially no RF plasma source power into the chamber, and introducing into the chamber a process gas comprising oxygen and carbon monoxide, until the polymer layer is removed from the surface of the photoresist mask.

Description

Plasma etch and photoresist stripping process with intermediate steps of chamber defluorination and wafer defluorination

Background technique

The performance of integrated circuits is continually being improved by increasing device switching speeds, increasing interconnect densities, and reducing crosstalk between adjacent conductors separated by intermetal dielectric (IMD) layers. Improved switching speed and reduced crosstalk can be achieved by using new dielectric thin-film materials as IMDs with low dielectric constants ("low-k materials"), such as porous organosilicate glass. Interconnection is increased by increasing the number of interconnecting conductive layers and reducing feature size (eg, line width, aperture). Connections between different conductors require high-aspect-ratio (depth-to-width) openings or "vias" through low-k materials. Such fine features (eg, feature sizes on the order of 45nm) require photoresists (used in photolithography) to be suitable for larger wavelengths. This photoresist tends to be thinner and more prone to the formation of defects, such as pinholes or streaks, during the dielectric etch process. This problem is addressed by using fluorocarbon chemistries during plasma etching to form narrow vias through low-k dielectric films. Fluorocarbon etch chemistries deposit protective fluorocarbon polymers on photoresist. The etch process typically ends when it reaches the bottom dielectric layer above the copper interconnect lines. This bottom dielectric layer typically acts as a barrier to the diffusion of copper atoms from the conductive lines, and is itself a low-k dielectric material (such as nitrogen-doped silicon carbide) and is typically extremely thin (on the order of hundreds of angstroms). After the barrier layer is exposed, the etch process is stopped and a deep and narrow (high aspect ratio) opening or via is formed. In preparation for the next process step, the photoresist is stripped from the wafer. This photoresist stripping process can be done in an ammonia-based plasma with bias power applied to the wafer, and in the same chamber where the etch process was previously performed, avoiding unnecessary wafer transfer steps and maximizing throughput change. The problem is that the photoresist stripping process causes the thin barrier layer at the bottom of the via made of low-k material to disappear.

One way to avoid this problem is to transfer the wafer into a dedicated photoresist ashing chamber before performing the photoresist ashing step. Unfortunately, however, this approach reduces throughput due to inherent delays in transferring wafers between reactor chambers.

Therefore, there is a need for a combination via etch and photoresist stripping process that can protect the thin barrier layer at the bottom of the via.

Contents of the invention

A plasma etch process comprising a plasma etch step through a photoresist mask on a workpiece, the etch step using a polymerized etch process gas generated in a plasma polymerized substance that etches step to build up a protective polymer layer on the surface of the photoresist mask, the process comprising the following steps in the same chamber after the etching step and before removal of the photoresist mask:

Removing residues comprising polymeric material from chamber surfaces including a ceiling of the chamber is accomplished by coupling RF plasma source power into the chamber while substantially not coupling RF plasma bias power into the chamber , and introducing a hydrogen-containing gas into the chamber until the residue is removed from the chamber surfaces;

Removal of the protective polymer layer from the surface of the photoresist mask is accomplished by coupling RF plasma bias power into the chamber while substantially not coupling RF plasma source power into the chamber, and will contain oxygen and carbon monoxide process gases are introduced into the chamber until the polymer layer is removed from the photoresist surface.

Description of drawings

Fig. 1 is the block flow diagram of implementing technology of the present invention;

Figures 2A, 2B, 2C and 2D show the film structure changes during each step of the process of Figure 1 in sequence;

Figure 3 shows a plasma reactor suitable for performing the process of Figure 1 and having a controller programmed to perform the process of Figure 1 .

Detailed ways

The plasma etching and photoresist stripping process of the present invention solves the problem of removing the bottom barrier layer of the low-k via via the photoresist stripping process. The present invention is based on the inventor's realization that the barrier layer removal problem during resist stripping is caused by the presence of fluorine-containing residues both on the interior surfaces of the chamber and on the wafer itself at the beginning of the resist removal step after etching . The photoresist stripping process releases fluorine compounds (and free fluorine) from the residue deposited during the etch process. The hydrogen present in the ammonia-based plasma used to strip the photoresist combines with the liberated fluorine compounds, resulting in highly reactive plasma etching of the low-k barrier layer. The process of the present invention eliminates this problem.

Referring now to FIG. 1, a photoresist mask is photolithographically defined on the top surface of the thin film structure (block 10 of FIG. 1). The film structure at this stage of the process is shown in Figure 2A. The thin film structure of FIG. 2A comprises an antireflective coating 12 covered by a photoresist layer 14 deposited in the step of block 10 of FIG. 1 , the photoresist layer 14 having photolithographically defined apertures 14a. A silicon dioxide-containing dielectric layer 16 is positioned below the antireflective coating 12, a layer of organosilicate glass 18 is positioned below the dielectric layer 16, and a thick porous organosilicate glass layer 20 (low-k dielectric material) is positioned over the organosilicate glass layer 18 below. A thin diffusion barrier layer 22 underlies the porous organosilicate glass layer 20 and is composed of a low-k dielectric material, such as nitrogen-doped silicon carbide, that prevents or blocks the diffusion of copper atoms. A copper wire 24 surrounded by an oxide layer 26 is located below the barrier layer 22 .

In the next step (block 30 of FIG. 1 ), a fluorocarbon or fluorocarbon process gas is flowed into the reactor chamber while plasma RF source power and plasma RF bias power are applied to operate at relatively low chamber pressures (e.g., mTorr level) to form a plasma. This condition is maintained in the chamber until a via 32 is opened through the thin film structure of FIG. 2B down to the top surface of barrier layer 22 . FIG. 2B shows the film structure after the step of block 30 in FIG. 1 is completed. During the etch step of block 30 of FIG. 1, a portion of the process gas forms a simple (high fluorine content) fluorocarbon etchant species for forming via holes 32 (FIG. 2B). Simultaneously, other carbon-rich fluorocarbon species are formed, which accumulate as a polymer layer 34 on the top surface of the photoresist layer 14 (FIG. 2B) and as a polymer layer on the interior surface of the reactor chamber ( not shown in Figure 2B).

In block 40 of FIG. 1 , prior to removal of the photoresist layer 14 , the step of removing the fluoropolymer from the reactor chamber surfaces without damaging or removing the fragile thin barrier layer 22 is performed. This is accomplished by removing the bias power on the wafer and then replacing the fluorocarbon or fluorocarbon process gas in the reactor chamber with ammonia. The plasma RF power source decomposes the ammonia sufficiently that the hydrogen from the ammonia treatment gas reacts with the fluorine and carbon atoms in the polymer on surfaces within the chamber, thereby removing the polymer from those surfaces. The RF bias power on the wafer is set to zero (or close enough to zero) to prevent the ammonia-generated plasma from reaching down the narrow vias and eroding the thin barrier layer 22 . As a result, this step can be performed long enough to ensure complete removal of accumulated polymer from interior chamber surfaces.

In block 42 of FIG. 1 , another step is performed prior to removing the photoresist layer 14: removing the fluoropolymer layer 34 from the top surface of the photoresist layer 14 without damaging or removing the easily accessible material at the bottom of the via 32. Damaged and thin barrier layer 22. This is accomplished by removing RF source power and applying RF bias power to the wafer. Furthermore, a process gas environment consisting of oxygen and carbon monoxide and free of any hydrogen- or fluorine-containing compounds is established within the reactor chamber. For example, while the order in which the steps of blocks 40 and 42 are performed can be reversed, if the step of block 42 is performed after the step of block 40, a suitable process gas environment is established as follows: complete removal of ammonia process gas, removal of RF plasma Source power, flow oxygen and carbon monoxide into the reactor chamber, and apply RF plasma bias power to the wafer. The lack of RF plasma source power limits (or prevents) the decomposition of oxygen molecules. The undecomposed oxygen does not attack the thin barrier layer 22 but attacks the polymer layer 34 on the surface of the photoresist layer 14 . The reaction of oxygen with polymer layer 34 removes polymer layer 34, resulting in the structure shown in FIG. 2C, while generating in the plasma some highly reactive (low carbon content) containing Fluorocarbons. The presence of carbon monoxide atoms in the process gas prevents this attack. The carbon monoxide atoms react rapidly, combining with the low carbon content reactive species produced in this step, converting this reactive species into a more inert carbon-rich fluorocarbon species that does not attack the vulnerable barrier layer 22 .

In the final step (block 46 of FIG. 1 ), the photoresist layer 14 is removed in the same process chamber. The photoresist stripping or removal step of block 46 is typically performed by removing oxygen and carbon monoxide process gases, introducing ammonia gas, applying RF plasma source power to the top electrode of the reactor, and applying RF plasma bias power to the wafer. Since all fluorine-containing deposits have been removed from the chamber surfaces and the wafer itself, the plasma environment is fluorine-free so that the hydrogen released in this final step (photoresist removal) cannot damage the exposed barrier layer 22 . This step also removes the antireflection coating 12 . The photoresist removal step of block 46 is performed for a sufficient time to ensure photoresist removal and result in the thin film structure shown in FIG. 2D.

In experiments we have performed with the process of Fig. 1 it was found that the material loss of the barrier layer 22 was 2nm or less, which is a negligible amount, thus allowing feature sizes on the wafer to be reduced to 45nm.

FIG. 3 shows a plasma reactor for carrying out the process of FIG. 1 . The reactor of FIG. 3 has a vacuum chamber 100 defined by cylindrical sidewalls 105 and a disc-shaped chamber top electrode 110 . An electrostatic chuck 115 supporting a wafer 120 includes an insulator layer 125 having an inner electrode 130 coupled to a DC chuck voltage source 135 . RF plasma bias power generator 140 is coupled to electrode 130 through impedance matching element 145 and insulating capacitor 150 . A VHF (eg, 162 MHz) RF source power generator 155 is coupled to the roof electrode 110 through an impedance matching element 162 . The impedance matching element may be a coaxially tuned stub that resonates near the frequency of the VHF generator 155, and the electrode 110 has a reactance that resonates with the plasma in the chamber 100 near the frequency of the VHF generator 155, as described in 2003 3 Disclosed in U.S. Patent No. 6,528,751 (assigned to the applicant) entitled "PLASMA REACTION WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA" by Daniel Hoffman et al., granted on April 4, the disclosure of which is incorporated herein by reference . The roof electrode 110 is also a gas distribution showerhead. The process gas flows into the chamber 100 through the inner region 160 of the gas distribution holes in the chamber top electrode 110 and the outer region 165 of the gas distribution holes. The gas plate 170 provides selected process gases and distributes the gas flow between the inner zone 160 and the outer zone 165 . The inner and outer electromagnets 180 , 185 control the plasma ion density distribution within the chamber 100 in accordance with the DC current applied thereto by the DC current source 190 . A vacuum pump 200 , regulated by a throttle valve 205 , establishes the desired vacuum chamber pressure in chamber 100 . The main controller 195 is programmed with a set of instructions 196 to perform a process sequence of the type shown in FIG. 1 and to this end controls the gas panel 170, the DC magnetic current source 190, the VHF source power generator 155, the RF bias power generator 140 and throttle valve 205 operation. In this case, the command set 196 programmed to the main controller 195 is as follows: (1) supply fluorocarbon process gas from the gas plate 170, apply bias and source power to etch vias through the low-k dielectric layer; (2) ) apply source power, turn off bias power and fill the chamber with ammonia gas from gas panel 170 to remove accumulated polymer from interior chamber surfaces; (3) apply bias power, turn off source power and fill the chamber with oxygen gas from gas panel 170 and carbon monoxide process gas to fill the chamber to remove deposited polymer from the wafer; and (4) remove photoresist from the wafer.

In a feasible embodiment, the conditions for performing the step of block 40 in Fig. 1 are as follows: the flow rate of ammonia to the shower head or chamber top electrode 110 is 300 sccm, the source power of the VHF generator is 400W, the chamber pressure is 10mT, and the inside The ratio of the flow rate of zone 160 to the outer zone 165 was 1.35, the DC current applied to the inner electromagnet only was 14A, and the processing time of this step was 30s. The conditions for performing the steps of box 42 are as follows: oxygen flow rate 100 sccm, carbon monoxide gas 50 sccm, source power 0, RF bias power 100 W (using 2 MHz and 13.56 MHz sources), chamber pressure 5 mT, inner and outer The ratio of the zone flow rate is 1.35, the DC current applied only to the inner electromagnet is 14A, and the processing time of this step is 60s.

Although the present invention has been specifically described by preferred embodiments, it should be understood that changes and improvements can be made to the present invention without departing from the true spirit and scope of the present invention.

Claims (14)

1. A plasma etching process for a workpiece having a low dielectric constant film exposed in the etching process, the process comprising: forming a photoresist mask on the top surface of the workpiece, the mask including openings defining hole locations to be etched; placing the workpiece in a plasma reactor chamber; The etching step was performed by introducing a polymeric etch process gas comprising a fluorocarbon species into the chamber, and coupling RF plasma source power and RF plasma bias power into the chamber to generate a plasma of: ( a) an etching substance used to etch holes in the workpiece aligned with the openings in the photoresist mask, and (b) during the etching step on top of the photoresist mask polymeric substances accumulating as polymer layers on surfaces and as polymer layers on interior surfaces of the reactor chamber; Before removing the photoresist mask: (a) removing residues comprising the polymer layer from chamber surfaces comprising the ceiling of the chamber, which is accomplished by coupling RF plasma source power into the chamber without biasing the RF plasma power is coupled into the chamber and ammonia is introduced into the chamber until the residue is removed from the chamber surfaces; (b) removing the polymer layer from the surface of the photoresist mask, which is accomplished by coupling RF plasma bias power into the chamber while not coupling RF plasma source power into the chamber, and introducing a process gas comprising oxygen and carbon monoxide into the chamber until the polymer layer is removed from the surface of the photoresist; and The photoresist is stripped from the workpiece. 2. The process of claim 1 wherein, during the etching step and the step of removing residue from chamber surfaces, the step of coupling RF plasma source power to the chamber includes passing through the chamber above the workpiece The top electrode of the chamber capacitively couples the RF power. 3. The process of claim 2 wherein, during said etching step and step of removing said polymer layer from said photoresist mask, the step of applying RF plasma bias power comprises switching HF and LF power At least one of is coupled to the workpiece. 4. The process of claim 2, wherein said RF plasma source power is at a VHF frequency of 50-300 MHz. 5. The process of claim 4, wherein the RF plasma source power is about 162 MHz. 6. The process of claim 3, wherein the RF plasma bias power comprises an HF portion of approximately 13.56 MHz and an LF portion of approximately 2 MHz. 7. The process of claim 1 , wherein the step of stripping the photoresist comprises coupling RF plasma source power and RF plasma bias power into the chamber, and introducing ammonia into the chamber until The photoresist is removed from the workpiece. 8. The process of claim 1, wherein said fluorocarbon species comprises a fluorocarbon species. 9. The process of claim 1 wherein the step of removing residues from chamber surfaces further comprises maintaining a low chamber pressure on the order of about 10 mTorr and a high flow rate of said ammonia containing on the order of about 300 sccm. 10. The process of claim 1, wherein the step of removing the polymer layer from the photoresist comprises maintaining a low chamber pressure of about 5 mTorr and an oxygen flow rate of about 100 seem and a carbon monoxide flow rate of about 50 seem, respectively. 11. The process of claim 1 , wherein said low dielectric constant film is a diffusion barrier layer beneath other film layers of said workpiece, and wherein said process includes stopping said etching when said diffusion barrier layer is exposed step. 12. The process of claim 11, wherein said other thin film layer comprises a low dielectric constant material comprising porous organosilicate glass. 13. The process of claim 12, wherein said diffusion barrier layer overlies a metal line of said workpiece, and wherein said diffusion barrier layer comprises a material capable of preventing diffusion of material from said metal line comprising nitrogen-doped silicon carbide. low dielectric constant material. 14. A plasma etching process comprising a plasma etching step through a photoresist mask on a workpiece, said etching step using a polymerized etch process gas generated in a plasma polymerized substance, said plasma polymerized substance A polymer layer builds up on the top surface of the photoresist mask and on the inner surface of the reactor chamber during the etch step, the process comprising removing the photoresist after the etch step Perform the following steps before masking: (a) removing residues comprising the polymer layer from chamber surfaces comprising the ceiling of the chamber, which is accomplished by coupling RF plasma source power into the chamber without biasing the RF plasma power is coupled into the chamber and ammonia is introduced into the chamber until the residue is removed from the chamber surfaces; (b) removing the polymer layer from the surface of the photoresist mask, which is accomplished by coupling RF plasma bias power into the chamber while not coupling RF plasma source power into the chamber, and a process gas comprising oxygen and carbon monoxide is introduced into the chamber until the polymer layer is removed from the surface of the photoresist.

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